Referring now to FIG. 1, a schematic illustration of a four-port switch 102 according to the prior art is depicted. In this example, port 2 is receiving stream B. A stream is a series of frames having a common source and destination. Stream B is destined for port 4 and is shown exiting port 4. Stream A, arriving on port 3, is bound for port 1 and is shown exiting port 1. The switch 102 may be connected to network devices that operate at different speeds—for example 10 Mbps, 100 Mbps, or 1 Gbps. If a network device connected to port 3 is operating at 1 Gbps and a device connected to port 1 is operating at 100 Mbps, port 1 may not be able to keep up with the data provided by port 3. The switch 102 may, instead of dropping frames, store frames in a queue 104. The queue 104 is finite, however, and after continued transmission at 1 Gbps to port 3, and only 100 Mbps out of port 1, the data will exceed the capacity of the queue 104 and frames will be dropped.
Once the queue 104 reaches a predetermined threshold 106, the switch 102 will instruct port 3 to issue flow control (if flow control is enabled) so that the queue 104 does not overflow and force the switch 102 to drop frames. The flow control may constitute providing backpressure or sending out a MAC PAUSE. While port 3 is paused and the queue 104 is decreasing, stream B can proceed from port 2 to port 4 without interference.